Technical Article

How to Reduce the Soil’s Resistivity

September 28, 2020 by Lorenzo Mari

Lowering the resistance of a grounding electrode is not always enough to reduce the soil’s resistivity. Reducing the resistivity of the soil around the electrodes can help achieve adequate resistance to the ground.

The elements that most affect the soil resistivity are moisture content, ionizable salts, and porosity. Water and ionizable salts combine to form an electrolyte, which is a conductor of electricity. Porosity is an indicator of the ability of the soil to retain the electrolyte.

Some methods to reduce the soil resistivity include:

  • Water retention
  • Chemical salts
  • Bentonite
  • Chemical-type electrodes
  • Ground enhancement materials

 

Water Retention

Most soils lose moisture when they receive direct sunlight. The sun heats the ground and causes the water contained in it to rise to the surface and vaporize, dispersing in the atmosphere. The longer the heating process, the drier the soil.

Excessive drainage can also quickly leach away the salts in the soil and dry out the deeper layers.

The water molecules ionize the minerals in the soil and cause them to become conductive. Without moisture, an electrical connection to earth is not possible. Figure 1 shows how the resistivity varies as a function of moisture content for various types of soil.

 

Soil resistivity and moisture content
Figure 1. Soil resistivity and moisture content

 

As observed, there is a strong association between the water content and the resistivity of soils. The best soil types require a minimum of 4% water (by weight), while the poorest require at least 14%.

Some areas of the world have soils with more than enough moisture. Others, however, have none. Deserts are mainly arid and have little to no soil moisture. A simple standard copper rod will not serve as a ground connection in these places unless water is added to the soil.

The soil may or may not retain an appropriate amount of moisture, according to its degree of porosity. This condition directly impacts both the distribution of the content of ionizable salts and the formation of the electrolyte. Improving the soil’s ability to retain moisture is an effective way to decrease its resistivity.

Where there is no moisture, providing it will achieve reasonable grounding. It is only required to moisten the soil near the grounding electrode. For the best results, the electrode/soil interface should be wet.

An infrequent but effective way to maintain soil moisture is to plant vegetables around the grounding electrodes. Vegetables retard runoff and retain irrigation water and salts in the soil, helping to keep the area moist and the salts dissolved. 

An irrigation system is also helpful in keeping the soil moist. The installation of an automatic moisture sensing and water supply system, in combination with a conventional water supply system, can precisely control the moisture content of a given soil.

 

Chemical Treatments

The use of ion-producing chemical compounds like sodium chloride, magnesium sulfate (epsom salt), copper sulfate (blue vitriol), and calcium chloride around the grounding electrodes, decreases the soil’s resistivity and the electrode’s resistance to ground.

The most widely-used chemical is magnesium sulfate. It is low-cost, has strong electrical conductivity, and has little corrosive effect.

Ordinary rock salt is cheap. Common salt (sodium chloride) is highly corrosive. This corrosive effect may cause nearby metal objects to deteriorate. Despite being an excellent conductor of electricity, its adverse effects remove it from the list of preferred chemicals.

The chemical treatment indirectly increases the diameter of the electrode by modifying its surrounding soil. When the soil is porous, the solution permeates quickly into a large volume of earth, making a large equivalent diameter, with quick results.  In contrast, when the soil is compact, the chemicals take time to spread, and results are produced more slowly.

A practical way of applying these compounds is through a circular trench excavated around the ground rod, preventing direct contact with the electrode (Figure 2). 

 

Soil treatment with a circular trench. Image based on a drawing from  IAEI.
Figure 2 Soil treatment with a circular trench. Image based on a drawing from  IAEI.

 

It can be beneficial to supply a little water through a pipe to accelerate the effect of the salts. The amount of water should be sufficient to keep the area  moist, but without washing away the salts.

The chemicals are gradually washed away by natural drainage through the soil and rainfall, requiring periodic replacements. The period for replacement varies depending on site conditions, but it may be years. An adequate maintenance scheme will ensure that chemicals will have long-lasting effects.

A particular characteristic of the chemical treatment is the reduction of seasonal variations of the resistance to ground. These variations come from the periodic drying and wetting of the soil.

Use caution, as local authorities may prohibit the use of chemicals if they are not considered environmentally friendly.

 

Bentonite

Adding bentonite to the soil reduces its resistivity and the ground resistance of the electrodes.

Bentonite is a fine-grained, highly plastic clay, formed by volcanic action. It may be used as soil replacement and filler material for electrical grounding in places with high resistivity. The conductive Bentonite clay is a sodium activated montmorillonite. Bentonite is chemically hydrated, innately stable, and retains its properties over time.

Bentonite absorbs moisture from the surrounding soil and swells up to several times its dry volume. It adheres to the surface of the grounding rods and cables laid in trenches, reducing the contact resistance and increasing their diameter artificially.

The resistivity of bentonite depends on the water content. The water inside the pores allows the electrical currents to move through the bentonite. The resistivity value is lower in the liquid state than in the plastic or solid state and is on the order of 250 Ω∙cm at 300% moisture.

In addition to reducing the resistance to ground of rods and cables, the moisture retention process of the bentonite compound protects against corrosion.

Bentonite performance is highly dependent on the amount of rainfall, soil moisture, and temperature at the site. In hot climates, the soil dries out, and the bentonite does not work as desired. It may separate from the electrodes, increasing the resistance to the ground.

 

Chemical Rods

Chemical rods are suitable for high resistivity soils — rock, mountain tops, sandy soil — and places with excessively high or low temperatures.

This type of rod is a tube filled with mineral salts distributed evenly.  It has holes along its length, allowing the entry of soil moisture. The moisture combines with the salts and dissolves them. The saline solution then seeps out through the holes and soaks into the surrounding soil, continuously conditioning a large volume around it.

The materials available are copper, stainless steel, and hot-dipped galvanized iron. Its length choices are the same as conventional rods: 240 cm (8 ft) and 300 cm (10 ft).

They may be installed by drilling holes in the ground, and, for rocky soils, manufacturers offer horizontal rods. It is customary to place a grounding enhancement fill around the rod to improve the interface with soil.

These rods also require maintenance. For this, they have a removable cap for inspection and chemical supply (Figure 3).

 

Chemical rod. Image courtesy of Lightning Eliminators
Figure 3. Chemical rod. Image courtesy of Lightning Eliminators

 

Grounding Enhancement Fill

Replacing all or part of the soil around an electrode with a low resistivity filler will facilitate the achievement of low ground resistance. The higher the percentage of earth swapped, the lower the ground resistance.

A grounding enhancement fill may have resistivities as low as 50 Ω∙cm (much lower than bentonite). It works in a trench, around a ground rod or substation grounding conductors, either dry or in a slurry.

The main properties are: constant resistance, low resistivity, maintain moisture, stability, low freezing point, resistance to leaching, non-corrosive, and maintenance-free.

 

Reviewing How to Reduce the Soil’s Resistivity

When the grounding electrode does not achieve a low enough resistance, another option is to reduce the resistivity of the soil. There are several methods to accomplish this: water retention, chemical salts, bentonite, chemical-type electrodes, and ground enhancement materials.

While all methods are effective, the selection will depend on the site’s particular conditions and the ability to carry out proper maintenance when required.